Chemical-mechanical planarization ("CMP") processes are frequently used to planarize dielectric layers in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a slurry on a polishing pad under controlled chemical, pressure, velocity, and temperature conditions. Slurry, solutions generally contain small, abrasive particles of silica or alumina that mechanically remove the surface of the wafer, and chemicals that react with the materials of the dielectric layers to enhance the removal of the molecules on the surface of the wafer. The polishing pad is generally a planar pad made from a relatively soft, porous material such as blown polyurethane.
CMP processes must accurately planarize the dielectric layer to a desired end-point. Several hundred microelectronic devices are typically fabricated on a single wafer by depositing layers of various materials on the wafer, and manipulating the wafer and the other layers of material with photolithographic, etching, and doping processes. In order to manufacture ultra-high density integrated circuits. CMP processes must provide a uniformly planar surface so that the geometries of the component parts of a die may be accurately positioned across the full surface of the wafer. Thus, it is important to accurately planarize the wafers to a desired endpoint across the whole wafer.
In the competitive semiconductor industry, it is also highly desirable to maximize the throughput of the CMP processes to produce accurate, planar surfaces as quickly as possible. The throughput of CMP processes is a function of several factors including the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate"), and the ability to accurately estimate the planarizing time to stop the process at a desired endpoint. A high polishing rate generally results in a greater throughput because it requires less time to planarize a wafer. Accurately estimating the planarization time is also important to maintaining a high throughput because planarization of a wafer is stopped once the estimated planarization time has elapsed to remove the wafer from the pad and measure the actual thickness of the wafer. If the thickness of the dielectric layer is not within an acceptable range, the wafer must be re-planarized until it reaches a desired endpoint. Such re-planarization of a wafer significantly reduces the throughput of current CMP processes, so it is very important to provide an accurate estimate of the planarization time.
One problem with current CMP processes is that they do not consistently produce a uniformly planar surface at the desired end-point because the wafer polishing rate may change over a number of wafers. Wafer polishing rates change for a number of reasons, and it is difficult to determine which one of the wafer polishing operating parameters must be corrected to bring them back to a desired level. Some of the wafer polishing operating parameters that affect the polishing rate include: (1) the downward pressure of the wafer against the slurry and pad; (2) the relative velocity between the wafer and the pad; (3) the chemical and abrasive characteristics of the slurry.; (4) the condition of the pad; and (5) the temperature of the slurry and the wafer. The downward pressure of the wafer and the relative velocity between the wafer and the pad are relatively easy to measure and indicate to the operator. The characteristics of the slurry, the temperature gradient across the wafer, and the condition of the pad, however, are difficult to ascertain. For example, a polishing pad will become less effective after planarizing a wafer because materials from the wafer and the slurry adhere to the surface of the pad and reduce the pad's ability to abrade the wafer. Polishing pads are consequently "conditioned" to bring them back to their optimal state for planarizing a wafer by abrading their surfaces with a diamond-embedded stone. Accordingly, if the polishing rate changes for a reason other than the pressure and velocity, the operator must guess whether the change was caused by the condition of the pad, the temperature gradient across the wafer, the effectiveness of the slurry, or some other reason that is not readily indicated on the control panel of the polisher. Therefore, it would be desirable to develop an apparatus and a method that indicates whether or not the polishing pad is the reason for a change in the polishing rate.
In addition to identifying which operating parameters have changed, a more fundamental problem with current CMP processes is that it is difficult to timely ascertain when an undesirable change in operating parameters has occurred. A change in the operating parameters is typically indicated by measuring the actual change in thickness of a number of wafers for a given planarizing time, and noting a significant change in the polishing rates over the number of wafers. Measuring the actual thickness of a wafer, however, is time-consuming because wafers are delicate and require sophisticated, clean-handling procedures. Thus, it would also be desirable to develop an apparatus and a method for indicating when an undesirable change in the operating parameters has occurred without having to measure the actual change in thickness of the wafers.